The present invention relates to an electric switching device for high current in an electrical power network. More particularly, the invention relates to a so-called high-speed circuit closer which is adapted to achieve a fast mechanical electric short circuit of at least one phase in a multi-phase network. The switching device is preferably intended to be used as arc eliminator in cubicle-enclosed switchgear in distribution networks and in transmission networks. Thus, the invention comprises both low, medium and high voltage. In accordance with the IEC standard, medium voltage means 1-72.5 kV whereas high voltage is  greater than 72,5 kV.
In arcs, caused by large fault currents, very large amounts of energy are released in the form of heat and radiation. In, for example, enclosed switchgear, with its limited space, these amounts of energy give rise to increases in pressure which may blast the enclosure. Such switchgear must therefore be equipped with space-demanding relief openings through which the heated gases are given a possibility to flow out. Further, the high arc temperatures cause the material in conductors and in switching equipment to melt and even evaporate. Burnable organic material may also be ignited when subjected to the high temperature and intense radiation of the arc. By decomposition of air (NOx) and evaporation of metals, the arc gives rise to poisonous gases. It is, therefore, common for such switchgear to be provided with devices for pressure relief in the form of evacuating channels, automatically openable hatches, etc. This means that such switchgear will be bulky and costly.
In industry, there has long been a great need to primarily prevent arcs from arising. In the second place, there has been a need to minimize the arc duration. In this way, material damage as a result of the heat and the pressure increase, built up during the duration of the arc, can be reduced. Also, the risk of personal injury and poisoning is reduced.
In case of a duration of the arc of about 30 ms, a switch-gear unit may be completely blown out. The pressure wave caused by the arc usually reaches its maximum even after 10-25 ms. To reduce the material damage, there is thus a need to limit the arc duration to about 10 ms. To reduce the risk of personal injury, still shorter arc durations should be aimed at. However, known circuit breakers have a considerably longer break-time. One common way of limiting the duration of the arc is, therefore, to rapidly switch the fault current to ground before the actual breaking operation.
A common problem in connection with a closer is that burns arise on the contact surfaces. Immediately before the switching elements come into contact with each other, arcs arise which partially melt the metal in the contact elements. This deteriorates the contact and causes development of heat. Known closers must therefore be overhauled at short intervals.
From, for example, DE-A-2623816, it is previously known to use, in gas-insulated, metal-enclosed switchgear, a fast grounding switch to extinguish a short-circuit arc between a high-voltage conductor and the grounded enclosure, to thus eliminate the risk of a dangerous overpressure building up. The grounding switch described in the publication is of single-phase design and is operated by a built-in explosive charge, the ignition of which is initiated by a sensor actuated by the arc. One disadvantage with such a grounding switch is that it must undergo a general overhaul or be replaced after one single operation. A further disadvantage is the handling and storage of explosives during such an overhaul.
From WO 97/45851, a high-speed circuit closer is previously known which, with a torsion-sprung contact device, brings a knife-shaped contact part into contact with a fork-shaped contact part. Both contact parts are housed in a container with insulating gas. The task of the known high-speed circuit closer is to achieve a rapid closing and prevent burns from arising on the contact parts. One disadvantage with this high-speed circuit closer is that a relatively large number of parts are included in the actual movement. The composition of the breaker is also relatively complicated. For optimum function, the torsion spring has to be adjusted carefully. A further disadvantage is that the closer only functions as a closer. Thus, it requires manual return to the insulating position. The spring must then be tensioned to be put into operational condition again.
The object of the present invention is to suggest ways and means of achieving a fast electric switching device, the break-time or make-time of which is less than about 5 ms. In its function as a grounding switch, a so-called arc eliminator, it shall prevent the occurrence of arcs at the moment of contact so as to avoid damage to the contact elements. It shall effectively brake the movable contact system during a closing operation. The switching device shall be able to carry out several closing and opening operations in rapid succession, also in the reverse order. The switching device shall manage both a high voltage and a high current. It shall have a simple and compact design which makes possible installation in conventional air-insulated switchgear without the above-mentioned disadvantages which are associated with prior art designs.
These objects are achieved according to the invention by an electric switching device according to the characteristic features described in the characterizing portions of the independent claims 1 and 6 and by a method according to the characteristic features described in the characterizing portions of the independent method claims 7 and 8. Advantageous embodiments are described in the characterizing portions of the dependent claims.
In high-voltage switchgear, grounding switches of two kinds are used, namely, working grounding switches and high-speed grounding switches. From a first aspect of the invention, the switching device is adapted to constitute a grounding switch of the latter kind. Such a fast grounding switch, a high-speed grounding switch, shall manage to ground the high-voltage parts also when these are energized and in case of large fault currents. In such a switching case, the contacts are subjected to full short-circuit current. In order thus to limit the contact burn-off and other function-reducing effects which are caused by the arcs which are usually created during the closing operation, bouncing movements between the contacts must be limited or completely eliminated. The bouncing movements are primarily dependent on the speed at which the contact parts butt against each other. The amplitude of the bouncing movement thus increases with the relative final speed between the contact parts. An important task of the high-speed circuit closer is therefore to achieve a low speed of the contacts at the moment of closing and an ability to damp the kinetic energy of the contacts.
From a second aspect of the invention, the electric switching device is adapted to constitute a circuit breaker. Usually, a circuit breaker must be able to handle the occurrence of an arc between the contacts. To this end, a conventional circuit breaker is equipped with means for extinguishing an arc thus arisen. However, the switching device according to the invention is so fast that no significant amount of energy has time to build up in the arc. The arc therefore expires by rapid separation of the contacts during zero crossing in the current.
The switching device according to the invention is arranged with a first contact part, a second contact part and a movable contact part displaceable therebetween. In the open position, the movable contact part rests at the first contact part. In the closed position, the movable contact part rests in contact with both the first contact part and the second contact part, whereby the current is passed through the movable contact part. In a first stage of the closing or opening movement, a rapid acceleration is imparted to the movable contact part. In a subsequent stage, the movable contact part moves at constant speed. In a conventional arc eliminator, a movable contact part usually moves under continuous acceleration during the whole movement, whereby a high speed of the movable contact part is achieved at the moment of closing. In a switching device according to the invention, where the movement is divided into an acceleration phase and a phase with a constant speed, a lower speed of the movable contact is obtained, for the same make-time, at the moment of closing.
The accelerating movement of the movable contact part is achieved with a so-called Thomson coil. When a current pulse traverses a flat-wound helical coil, the coil forms a variable magnetic field. In an adjacent metallic object, eddy currents are formed which, in turn, create a variable magnetic field directed in the opposite direction. For large current pulses, a strong repulsive force is formed between the coil and the adjacent metallic object. However, the force decreases rapidly with the distance. According to the invention, the movable contact part is designed as a sleeve of metal with a flange arranged at one end. The sleeve surrounds the first contact part and is arranged, in its open position of rest, such that the flange makes contact with a helical, flat coil arranged around the first contact part. When a current pulse traverses the coil, a strong repulsive force is thus formed between the coil and the flange, which force pushes away the sleeve against the second contact part.
For the breaking movement of the switching device, a Thomson coil is arranged in a similar manner. Secured to a housing of insulating material, surrounding the contacts, a helically wound flat coil is arranged. Spring-loaded against this coil, a metallic ring surrounding the sleeve is arranged. When a current pulse traverses the coil, a repulsive force arises which pushes the ring in a direction away from the second contact part. The ring and the flange of the sleeve are arranged such that the ring hits the flange. Upon this contact, the ring transfers its kinetic energy to the flanged sleeve, causing the sleeve to resume its open position of rest.
Between two poles with different potential, an electric field arises. The appearance of this field is largely dependent on the shape of the electrodes. If the electrodes are pointed or contain edges or irregularities, concentrations in the electric field arise. Such concentrations in the field may give rise to partial discharges. These discharges may, in turn, be the embryo of arcs. The switching device according to the invention is arranged with round electrodes, whereby concentrations in the electric field are avoided. As a result thereof, it has been possible to reduce the distance between the electrodes in case of full insulation. A smaller distance gives less energy requirement for moving a contact element which brings about contact between the electrodes. Due to the smaller distance it also takes less time to bring about contact. At high voltages, the contacts are arranged enclosed in a container filled with insulating gas, the insulating gas being utilized to further reduce the distance between the contact parts.
When two contacts approach each other, a small arc arises at the moment of contact. This is dependent on the current but also on the bouncing movement which arises between the contacts during the impact. The energy which causes these movements is dependent on the speed squared. This means that a reduction of the speed is exceedingly favourable for reducing the occurrence of bouncing movements. In case of a sliding contact, where the contact parts are moving in parallel with the contact surface, bouncing effects also occur in that a transverse force is imparted, upon impact, to the contact parts, which force sets the contact parts in oscillation. The oscillation causes the contact parts to alternately be in contact with each other and alternately be at a distance from each other. During the short time during which the contact parts are separated from each other, an arc arises which causes damage to the contact surfaces.
According to the invention, the harmful arcs are eliminated by means of a plurality of spring fingers. These are made of a conducting material with a high mechanical yield point. The spring fingers are placed in contact with the second contact part and are adapted, upon oscillation, to exhibit a high mechanical resonant frequency. When the movable contact part is brought to rapidly slide against the second contact part, it first hits the contact fingers, which are thrown sideways. By this force, the spring fingers are set into oscillation such that a bouncing movement arises. The spring fingers are dimensioned to obtain a high resonant frequency. On its way, the movable contact part knocks against a plurality of fingers, which are all brought into oscillation. The spring fingers are struck at different times and have different resonant frequency. This means that the phase difference between the oscillation of the different fingers will be random. Since the oscillation frequency is high, some finger will always be in contact with the movable contact part. This means that arcs do not arise and when the movable contact part has assumed the closed position, also the vibration of the contact fingers has decreased, enabling the grounding switch to carry the current,
As contact-making member between each of the first and the second contact part and the movable contact part, one or a plurality of tapes of helically wound wire are used. In conventional switching devices, this type of contact member occurs at the corresponding contact between the first contact part and the movable contact part. However, it is not known to arrange such a contact member for providing contact between correspondingly the second contact part and the movable contact part. Thus, conventional switching devices exhibit a fixed contact part, corresponding to the second contact part, which has contact fingers.